Rapidly solidified metal powders are in the form of fine crystal grains and can be adapted to contain alloy elements to supersaturation, so that the extrudates and sintered materials prepared from rapidly solidified powders are superior to materials prepared by melting in characteristics and have attracted attention as materials for making machine parts.
The methods of producing rapidly solidified metal powders include the rotary drum method as disclosed in Examined Japanese Patent Publication HEI 1-49769. With this method, a rapidly solidified metal powder is prepared by rotating a cooling drum having a bottom and containing a cooling liquid to centrifugally form a cooling liquid layer over the inner periphery of the drum, and injecting a molten metal into the cooling liquid layer to divide the metal by the cooling liquid layer in a swirling motion.
On the other hand, U.S. Pat. No. 4,787,935 and U.S. Pat. No. 4,869,469 disclose methods and systems for producing a metal powder by atomizing a molten metal stream into spherical molten droplets and supplying the droplets to a swirling downward flow of cooling gas within a cooling cylinder for cooling and solidification.
The rotary drum method is adapted for a so-called batchwise operation and therefore has the problem of being low in productivity. Furthermore, the speed of rotation of the cooling drum, which is limited, poses the probelm in that it is difficult to give an increased flow velocity to the cooling liquid layer and to obtain a fine powder.
On the other hand, the production methods of the U.S. patents are adapted to continuously prepare a fine powder of 0.1 micrometer in size to a coarse powder of about 1000 micrometers. With these production methods, however, the cooling rate is limited to about 10.sup.2 -10.sup.7 .degree.C./sec and fails to achieve a sufficient rapid cooling effect. Further because the molten droplets encounter difficulity in undergoing a swirling motion in the central portion of the swirling cooling gas flow and are cooled at a reduced rate, there arises the problem that the quality of the powder produced is liable to involve variations. Additionally, the cooling cylinder needs to have a considerably large size to form therein a swirling cooling gas flow which is suitable for cooling the molten droplets. This poses another problem in that the methods are difficult to practice readily in view of the installation space and equipment cost.
An object of the present invention, which has been accomplished in view of the above problems, is to provide a method of producing metal powders which is less likely permit variations in cooling rate, ensures rapid solidification at a great cooling rate and readily gives fine particles, and a production apparatus which is suitable for practicing this method.